Hobbybotics AD595 Thermocouple Breakout V1.0

Introduction

I designed a breakout board for the Analog Devices AD595-AQ Cold-Junction Compensated Type-K Thermocouple interface. This breakout board is an alternative to the MAX6675 breakout from a previous project. The difference is the AD595 can be operated from a single analog pin of a micro-controller. This project details a prototype and finished PCB design along with some sample Arduino sketches for demonstration.

The specifications for the AD595 are:

Works with any Type-K Thermocouple

Accuracy of +/- 3 degrees Celsius (AD595-AQ)

Has a temperature range up to 500 degrees Celsius in 10 mV/C degree increments

Operating voltage: +5V (see datasheet for dual supply operation)

Single analog input (ADC)

A thermocouple is a device consisting of two different metal alloy conductors that produce a voltage that is proportional to a temperature difference between the joined end of the two conductors. Dissimilar metals are chosen to maintain a predictable and repeatable temperature range profile.

Basically, two different metals are chosen in order to be able to measure a certain temperature range with an emphasis on stability. These two metals are joined together at one end and when the junction is heated or cooled a small voltage is produced that can be correlated back to a temperature value.

Thermocouples are available in many different types of metals and/or calibrations with the most common being of type J, K, T and E. Each calibration has a different temperature range but, the maximum range varies with the diameter of the wire used to create the thermocouple. Refer to the following document for temperature limits based on wire size (Thermocouple Reference Guide).

Below is a picture of a Type-K Thermocouple:

Here are some pictures of the prototype breakout board:

The AD595 as used in this project is designed to work with a Type-K Thermocouple which is made up of chromel (90% nickel and 10% chromium) and alumel (95% nickel, 2% manganese, 2% aluminum and 1% silicon).

Common uses are in HVAC systems, kilns and heating controllers.

The breakout board in the above pictures is a prototype design. The finished PCB is visualized in the below pictures:

The AD595 library provides Celsius and Fahrenheit temperature functions. The LCD I2C library provides an interface to an HD44780 parallel LCD using an MCP23008 I2C I/O Expander. The LCD library is not needed if you intend to only use the serial monitor for output. The MCP23008_SDA and MCP23008_SCL pins of the I/O Expander are connected to the SDA (Analog 4, pin 27) and SCL (Analog 5, pin 28) on the Arduino. The BKLT pin is connected to one of the I/O pins on the MCP23008 but, it can be connected to one of the PWM pins on the Arduino if you want to vary the LCD brightness with PWM.

Connect the below pins of the AD595 to the Arduino. DO can be connected to any of the analog pins A0-A5.

Pin

Function

VCC

+5V pin

DO

Signal pin. Outputs a voltage between 0 and 5V that correlates with the measured temperature. Can be connected to any of the analog pins (ADC).

GND

Ground pin

The generated voltage on the analog pin is linear with temperature thus, it easy to convert the reading to a temperature value. The AD595 outputs 10mV/C degree Celsius i.e. 1V = 100C, 2V = 200C…5V = 500C. The maximum temperature with a voltage reference of 5V is 500C.

The conversion formula is simple and probably looks familiar to some of you:

celsius = (5.0 * analogRead(pin) * 100.0) / 1024.0;

fahrenheit = (((celsius * 9) / 5) + 32);

The first example sketch prints temperature in Celsius and Fahrenheit to the Serial Monitor:

The second sketch prints temperature in Celsius and Fahrenheit to an I2C connected LCD:

Below is a picture of the output from the LCD test sketch with the thermocouple disconnected and connected. The Alarm LED will light if an error is detected with the thermocouple. This error can result from a detached or faulty thermocouple.

Faulty operation:

Normal operation:

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Disclaimer

This example shows hardware and software used to implement the design. It is recommended the viewer use sound judgment in determining and/or implementing this example for any particular application. This example may include information from 3rd parties and/or information which may require further licensing or otherwise. Additional hardware or software may be required. Hobbybotics or any affiliates does not support or warrant this information for any purpose other than a design example and takes no responsibility for any mishaps (none being implied).